Object window.opener.location.href is malfunctioning in IE8

"Error: 'window.opener.location' is null or not an object" is what I get when I try to do the following javascript:

  var UrlAddonString =  "&JobExtensionSubmissionFlag=1&StringModsToPlacementRecord=An effective date of " + document.getElementById("CommissionEFDate").value + " has been added to the placement record.|";
  
  window.opener.location.href = window.opener.location.href + UrlAddonString;

This worked perfectly fine in IE7, why does it not work in 8?